in contrast to conventional steel spring suspensions, pneumatic suspension systems have significant advantages and are therefore increasingly being used on commercial vehicles, such as trucks and buses, and also oh passenger cars, preferably heavy passenger cars, such as luxury sedans and SUVs. Thus, a pneumatic suspension system allows level control independent of loading since the current state of loading can, in each case, be compensated for by adapting the bellows pressure in the spring bellows of the pneumatic springs. Owing to the progressive spring characteristics of the pneumatic springs, a pneumatic suspension system likewise offers particularly reliable contact between the roadway and the wheels and a comfortable response during the compression and rebound of the wheels.
Another advantage of pneumatic suspension systems is that the ground clearance of the relevant road vehicles can be modified if required, e.g., increased for off-road use and reduced for high-speed travel on the freeway. In the case of commercial vehicles, there is the additional factor that the vehicle body can be lowered or adjusted to a suitable height for loading and unloading. Thus, the vehicle chassis of a truck or trailer with pneumatic suspension can he lowered to set down an interchangeable platform, for example, and raised again to pick it up. To facilitate loading and unloading, the loading surface of a truck can likewise be adjusted to the level of a loading ramp by lowering or increasing the bellows pressure at the rear axle. In the case of buses with pneumatic suspension, the vehicle body on the passenger loading/unloading side (curbside) can be lowered by releasing the compressed air from the spring bellows on the curbside and then raised again by filling the spring bellows in order to make it easier for the passengers to get in and out.
The basic construction of a pneumatic suspension system of the general type under consideration is known from DE 198 35 491 C2 and DE 100 04 880 A1, for example.
The pneumatic suspension system described in DE 198 35 491 C2 has a plurality of spring bellows, which can be connected by means of connection lines that are each provided with a level control valve to a main pressure line, and can be shut off with respect to same. The level control valves are each designed as 2/2-way solenoid switching valves, which are closed in a first position (rest position) and open in a second position (actuated position). The main pressure line can be supplied with air via a supply line provided with a compressor, an air dryer and a check valve, and can be vented via a vent line branching off between the compressor and the air dryer and provided with a discharge valve. The discharge valve is designed as a pressure-controlled 2/2-way switching valve, which is closed in a first position (rest position) and open in a second position (actuated position). The pilot valve associated with the discharge valve is designed as a 3/2-way solenoid switching valve, which connects the pneumatic control line to the environment in a first position (rest position) and to the main pressure line in a second position (actuated position).
In a first embodiment according to FIG. 1 of DE 198 35 491 C2, a throttle valve designed as a pressure-controlled 2/2-way switching valve, which is closed in a first position (rest position) and open with a throttle cross-sectional area in a second position (actuated position), and the pneumatic control input of which is connected to the pneumatic control line of the discharge valve, is arranged in a line segment parallel to the check valve. While air is being supplied to the main pressure line, the throttle valve is open, as is the discharge valve owing to the pilot valve, wherein the throttle cross-sectional area limits the air mass flow and causes it to expand ahead of the air dryer, thereby increasing moisture absorption by the compressed air from the air dryer and thus improving the regeneration thereof. In a second embodiment according to FIG. 2 of DE 198 35 491 C2, the discharge valve and the throttle valve are combined in a common pressure-controlled 4/2-way switching valve.
The pneumatic suspension system according to DE 100 04 880 A1 differs from that of DE 198 35 491 C2 in that a check valve is arranged between the compressor and the air dryer and, instead of the check valve and throttle valve being connected in parallel, a restrictor is arranged in the supply line after the dryer in the air admission direction. Moreover, the discharge valve then has a pressure limiting function and a check valve, which is activated in the second position (actuated position). Moreover, the pneumatic suspension system according to DE 100 04 880 A1 has a pressure accumulator, which can be connected by means of a connection line provided with an accumulator valve to the main pressure line and can be shut off with respect to same. In a first embodiment according to FIG. 1 of DE 100 04 880 A1, a high-pressure discharge valve designed as a 2/2-way solenoid switching valve is additionally provided, by means of which, if required, compressed air is discharged into the environment from the main pressure line while bypassing the air dryer. In a second embodiment according to FIG. 2 of DE 100 04 880 A1, a throttle valve with a controllable throttle cross-sectional area is arranged after the discharge valve in the venting direction. The throttle valve enables the air mass flow, which flows off into the environment during the venting of spring bellows, to be limited. As a result, the lowering speed of the vehicle body, e.g., at one vehicle axle or on one vehicle side, is controlled.
In Applicant's DE 42 43 577 B4, in contrast, a pneumatic suspension system of a motor vehicle is described in which a first control valve designed as a 3/2-way solenoid switching valve, by means of which a plurality of connection lines, each provided with a level control valve and leading to the e spring bellows of an associated pneumatic spring, can be connected to a pressure source, e.g., a pressure accumulator, or a pressure sink, e.g., the environment, has arranged after it in the air admission direction a second control valve, which is designed as a 2/2-way solenoid switching valve. In a first position (rest position), the second control valve is open without throttling and, in a second position (actuated position), is open with a throttle cross-sectional area. By actuation of the second control valve, the pneumatic suspension system can thus be switched between rapid admission of air to and venting of air from the spring bellows and slow air admission to and venting of air from the spring bellows. However, the throttle of the second control valve can be configured for only a number of spring bellows, i.e., for slow admission of air to and venting of air from two or four spring bellows for example.
Finally, DE 102 23 405 134 discloses a pneumatic suspension system that largely corresponds to that described in DE 198 35 491 C2, but, as in the pneumatic suspension system according to DE 100 04 880 A1, a pressure accumulator is provided, which can be connected by means of a connection line provided with an accumulator valve to the main pressure line and can be shut off with respect to same. In a first embodiment according to FIG. 1 of DE 102 23 405 B4, the discharge valve is designed as a 2/2-way solenoid switching valve, and in that a throttle valve with a controllable throttle cross-sectional area is arranged in the line segment parallel to the restrictor instead of a switching valve provided in one position with a constant throttle cross-sectional area. By means of the limited possibility of adjustment of the throttle cross-sectional area, the air mass flow flowing in or out via the air dryer during admission of air to and venting of air from spring bellows can be regulated and hence the raising and lowering speed of the vehicle body can be controlled locally, e.g., at one vehicle axle or on one vehicle side. However, a throttle valve with a controllable throttle cross-sectional area is a complex component involving a high outlay on production and is correspondingly expensive and fault prone.
Fundamentally, therefore, a problem with known pneumatic suspension systems is that the air mass flow during admission of air to and venting of air from the spring bellows and hence the raising and lowering speed of the vehicle body can be controlled or varied only inadequately. Whereas only relatively low air mass flows are required in the level control function and to compensate for leakage losses, relatively high air mass flows are passed into the relevant spring bellows or discharged therefrom for rapid lowering and raising of the vehicle body. In the hitherto known pneumatic suspension systems, this is possible to only an inadequate extent and in combination with functional disadvantages or functional limitations or only with a high equipment outlay.